1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type, as schematically illustrated in FIG. 1, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 having an upper portion 2a and a lower portion 2b and a first rack and pinion gear T.sub.1 including a first pinion 5 and a first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a, 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 induced by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear T.sub.2 including a second rack tooth portion 6b and a second pinion 17, a speed-reduction gear R, and a switching clutch 16 so that the driving force of the motor 13 is transmitted through the speed-reduction gear R, the switching clutch 16 and the second rack and pinion gear T.sub.2 to the rack 6 so as to assist the steering operation of the steering wheel 1 induced by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operations of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured.
However, the power steering system as constructed above has the following problems. Specifically, when the travelling speed of the vehicle increases above or decreases below a prescribed level, the control unit 9 operates to deenergize or energize the switching clutch 16 to interrupt or enable the transmission of the assisting force from the motor 13 to the rack 6 so as to switch the steering system into a manual steering mode without any power assist or into a power steering mode with power assist. In this case, if the speed sensor 3 has failed or an electrical line or wiring connecting the speed sensor 3 to the control unit 9 is broken for some reason, no output signal is fed to the control unit 9 so that the motor 13 continues to operate with the clutch 16 connected. As a result, the steering system is continuously held in a power steering mode even when the vehicle speed exceeds the prescribed speed level. Such a situation is unfavorable from the standpoint of safety in driving of the vehicle.